1. Field of the Invention
This invention relates generally to sheet registration, and more particularly to a method and apparatus for locating a sheet moving in a path at a registration position in the path.
2. Description of the Prior Art
It is a common practice in modern reproduction equipment such as printers or copier/duplicators, to deliver discrete sheets seriatim to a station at which the sheets are registered and a function is performed thereon. The function may be, for example, printing an image on the sheet or obtaining an image of information contained on the sheet to enable a copy thereof to be reproduced. In either case proper location of a sheet in the registration station is essential to obtaining accurate image reproduction. As efficiency of operation or speed of the reproduction equipment is increased in order to improve productivity, registration of the sheets becomes more difficult (e.g., must be accurately carried out in a shorter span of time), but no less critical.
A recent innovative apparatus for use with electrophotographic copier type reproduction equipment improves productivity by enabling the copier to make multiple copy sets of a multi-page document in page sequential order. The apparatus which is described in Belgian Pat. No. 835,568 (issued May 13, 1976 and U.S. Pat. No. 4,076,408 (issued Feb. 28, 1978 in the name of Reid et al, and assigned to Eastman Kodak Company), delivers discrete pages of the document seriatim from a document stack to the copier, whereat copies of the pages are made in the order received, and returns the pages to the stack. Because the document pages are continuously circulated to and from the stack until the desired number of copy sets are made, this apparatus is called a recirculating document feeder.
The document copying cycle utilizing the recirculating document feeder is more particularly described as follows: With the document pages stacked in page sequential order, information side up, in a supply hopper of the recirculating document feeder, successive pages of the document are removed from the bottom of the stack and fed seriatim, information side down, to the platen of the electrophotographic copier. The recirculating document feeder has a base plate overlying the platen to guide the pages in a travel path across the platen. Each page, in turn, is stopped and registered on the platen by registration gates located transverse to the page travel path across the platen and extending through openings in the base plate. The registered page is exposed by an illumination source within the copier to obtain an image of the page. The image is processed by the copier to make a copy of that page. After each page is exposed, the registration gates move out of the travel path and the page is fed back to the hopper and deposited on top of document pages remaining in the hopper. Recirculation of the discrete pages continues until the desired number of complete copy sets of the document have been made.
While registration gates as shown in the Belgian Patent have proven effective for proper page registration, other incidental aspects of their operation under certain conditions do not perform entirely in an optimum desired manner. For example, when a document page is slightly mis-registered (i.e., not in engagement with all of the registration gates), the openings in the base plate through which the non-contacted gates extend form a light trap. This causes an image of the opening to appear as an objectionable black artifact on the copy of the document page. Furthermore, the lead edge of the document page is not under control when it engages the gates. If the page is not flat against the platen during exposure, the image of the information on the page in the non-flat area may not be in focus. Additionally, the lead edge of light-weight document pages may be damaged as the page is driven into the registration gates, and heavyweight document pages may interfere with retraction of the registration gates out of the travel path after exposure.